The spatio-temporal distribution of nutrients and primary production processes are fundamental to understand the functioning of marine ecosystems. In this regard, the quantification of the new and regenerated primary production is highly relevant. In this work, the new production was estimated from the measurement of the uptake of nitrate by the phytoplankton by incubating samples enriched with stable isotopes of nitrogen (15NO3) together with the measurement of temperature, salinity, pH, chlorophyll-a and the measure of the cell abundance of the major phytoplankton groups. The experiments were carried out in cape Dos Bahías (CBD) (46°S) (open water environment without anthropogenic influence and depths greater than 50 m) and Bahía Nueva (BN) (42°S) (semi-enclosed sea environment with possible anthropogenic influence and depths less than 10 m). Temperature, salinity and pH were similar in the 2 analyzed ecosystems. The average concentration of nitrate, phosphate and silicate in BN was 0.19 ± 0.12 µM, 0.85 ± 0.09 µM and 1.77 ± 0.23 µM respectively. Comparatively, nitrate and silicate concentration in CDB was higher (0.86 ± 1.28 µM and 3.69 ± 1.07 µM respectively), while the phosphate concentration showed lower values (0.66 ± 0.20 µM). The concentration of chlorophyll-a was not associated with the nitrate uptake. In CBD, dinoflagellates and cryptophytes were the dominant groups and their abundance was positively correlated with the uptake of nitrate. In BN the diatoms dominated and were not associated with this parameter. We conclude that the new production in surface waters was similar for both environments and higher than in the sub-surface layer.